As someone who tested this extensively, it really needs to be treated like a major OS upgrade on a computer. Back up important info(pictures, etc) without using icloud, do a full format(DFU restore), and reinstall from scratch.
I have an iphone 6 and 6s here that are still flying along great on ios 11. And i had previously had so many problems with the 6S that i thought this upgrade really was awful. It took until 11.1 or so to truly be ok, but the performance increase from doing a true "clean install" like i would with windows or OSX was really night and day.
Combine that with a cheap battery replacement from your local Apple Store and your 6 should be great for another couple of years, I'd predict at least through iOS 12.
Historically speaking, we would expect the 6 to be supported by iOS 12, possibly by iOS 13 and unlikely by iOS 14.
The current trend for current iOS devices is five generations of software support with the iPhone 4S, 5 and 5S all getting five years.[1] The only recent exceptions are non-flagship models that used "old" parts at the time of release (iPhone 5C and iPad mini 1) which only got four years.
Yep, it's my backup/testing device. It actually runs surprisingly great, i'd say it's still a better experience than an equivalent-aged android phone honestly, even like a nexus 6
I have an iphone 6 and 6s here that are still flying along great on ios 11. And i had previously had so many problems with the 6S that i thought this upgrade really was awful. It took until 11.1 or so to truly be ok, but the performance increase from doing a true "clean install" like i would with windows or OSX was really night and day.